Cisco Systems has issued a range of security advisories giving details of 11 vulnerabilities in IOS, the operating system on which many of its products run.

One of the vulnerabilities, described as "highly critical," could lead to a hacker compromising the affected system or launching a denial-of-service attack against it. The advisories, issued Wednesday, are part of Cisco's twice-yearly schedule of security updates for IOS.

The highly critical vulnerability affects IOS version 12 devices running SIP, a protocol used by many businesses to set up and tear down voice and video calls. IOS version 12 is widely deployed.